1). Field of the Invention
The present invention relates to a slurry for use in chemical-mechanical polishing of a metal film on a semiconductor substrate.
2). Discussion of Related Art
The manufacture of integrated circuits involves the fabrication of multiple electronic devices such as transistors, diodes and capacitors in and on a silicon or other semiconductor wafer, and then interconnecting the devices with metal lines, plugs and vias.
During the manufacture of an integrated circuit, a number of layers of different materials are alternately deposited on one another and then partially removed. For example, during the formation of metal lines, a metal layer may be blanket deposited over the entire wafer so that metal of the metal layer covers higher areas on the wafer and fills trenches between the higher areas. The metal layer is then partially removed so that the higher areas are exposed and metal lines are left behind in the trenches.
One technique for removal of layers on a wafer is known in the art as xe2x80x9cchemical-mechanical polishingxe2x80x9d. In a chemical-mechanical polishing operation, a chemical-mechanical polishing slurry is applied over the metal layer which serves both a chemical and a mechanical function.
Chemically, a slurry of the above kind usually includes an oxidizer which oxidizes the metal layer by removal of electrons therefrom. An easily removable oxidized film is so formed by an upper portion of the metal film.
Mechanically, a slurry of the above kind also includes an abrasive such as silica (SiO2) or ceria (CeO2). The purpose of the abrasive is to abrade the oxidized film when a polishing pad is pressed against and moved over the film, and so remove the film.
Once the oxidized film is removed, the metal is again oxidized to form another oxidized film which is again removed utilizing the abrasive. The process is continued until the metal layer is removed to a required depth.
Slurries containing oxidizers may be unstable due to breakdown of the oxidizer over time resulting in unstable polish processes or production of toxic components. Replenishing and stabilizing these oxidizers are an added expense to be avoided. In addition, the spent slurry may have disposal issues and slurry oxidant requiring no waste treatment is desirable.
Another problem associated with the use of common chemical-mechanical polishing slurries is that they usually have pH values which are very low. Slurries having pH values which are very low are corrosive and may be the cause of damage to polishing equipment used in a chemical-mechanical polishing operation.
A further problem with conventional chemical-mechanical polishing slurries is that they cause xe2x80x9cerosionxe2x80x9d, xe2x80x9cdishingxe2x80x9d and xe2x80x9crecessxe2x80x9d during polishing. Erosion is the preferential loss of a layer due to high metal pattern density, and dishing is the topography developed in a wide metal line due to the metal being preferentially polished in the middle of the line with respect to the edge of the line. Recess, is the uniform loss of metal within a structure due to chemical activity and is typically dominant in narrow metal lines. Erosion, dishing and recess are undesirable as a planar surface is usually required.
The invention provides a chemical-mechanical polishing slurry comprising a liquid, cerium ions as an oxidizer, an abrasive, and a pH increasing substance. The cerium ions are in the liquid in a quantity equal to the inclusion of at least 0.02 molar ammonium cerium nitrate in the liquid. The abrasive is also included in the liquid. The liquid, the cerium ions and the abrasive jointly have a first pH value. The pH increasing substance increases the first pH value to a second pH value above 1.5.